Chapter 2
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Studies of Vanadate-Organic Ligand Systems Using Potentiometry and NMR Spectroscopy Lage Pettersson and Katarina Elvingson Department of Inorganic Chemistry, UmeåUniversity, SE-901 87 Umeå, Sweden
The aqueous speciation in some vanadium-organic ligand systems of biochemical interest has been determined by combined potentiometric (glass electrode) and quantitative V NMR data. Data (25 °C, 0.600/0.150 M Na(Cl) medium) have been collected over wide pH, [V], [ligand]/[V] ranges and evaluated using the least squares computer program LAKE, which is capable of simultaneously treating multimethod data. The results from the following systems will be reported: i) vanadate-dipeptides (alanylhistidine / alanylglycine / prolylalanine) ii) vanadate-nucleosides (adenosine / uridine with and without imidazole present) and iii) vanadate-maltol. Formation constants are tabulated for both the inorganic vanadate and the vanadate-organic ligand species. Distribution diagrams are used to illustrate equilibrium conditions. Some structural remarks are given. 51
The present paper is a summary and extended discussion of earlier work by the authors (1-6) on the aqueous equilibria of some biologically important vanadium(V)-organic ligand systems. The ligands studied (Figure 1) were chosen because of their proposed biogenic or therapeutic action when complexed to vanadium. As knowledge of the complete speciation and accurate pH-independent formation constants in the systems studied was limited prior to our work, the contribution is significant. Combination of potentiometry and V N M R spectroscopy is a powerful method for determining the complete speciation and has therefore been used. However, special precautions must be adopted. These will be discussed and commented upon in the Experimental section. To obtain accurate pH-independent formation constants of the vanadate - organic ligand species, the subsystems must be well known for the specific experimental conditions. A firm knowledge of the hydrolysis of V(V) is of special importance since the inorganic vanadate speciation is strongly dependent on the ionic medium. 5 1
30
©1998 American Chemical Society In Vanadium Compounds; Tracey, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
31
COO I
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N(D
I
or N
-CH -CH-NH 2
1
N(1) a 1 2b 3 4C 6d N H C—CH—CO—NH—CH—CH—[T^ \ 7
, 3
+
3
I
s
NH
J
^
5
l
2
COO"
I s^-N H N(3)
—
RN(3)
Histidine (His)
Alanylhistidine (AH)
a 1 2b 4 H C—CH—NH—CO—^ 3
J
3
COO"
j) Nf H 2
Prolylalanine (PAH)
Maltol (MaH)
Adenosine (AdH)
a
a 1 2b 3 4c 5 _ H C—CH—CO—NH—CH—COO 3
I .
NH
+ 3
Alanylglycine (AGH)
Imidazole (ImH)
Uridine (UrH ) 2
Figure 1. Ligands studied.
In Vanadium Compounds; Tracey, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
32 For a more complete description of the systems, including references to the literature, and for experimental details, the reader is referred to References 1-6 and citations therein. Experimental In equilibrium analysis, the combination of potentiometric titrations and quantitative V N M R measurements on solutions with accurately known pH values is a powerful method for determining the speciation. However, an ionic medium should always be used. When anion equilibria are being studied, the concentration of the cation in the medium should be kept constant. The present studies were performed in 0.600 or 0.150 M Na(Cl) at constant temperature (25 °C) and over a wide pH range. Buffers were not used by two reasons. They impose restrictions upon pH and some buffers can even interact with the system under study. Instead, pH was adjusted with HC1 or NaOH. Interactions with undesirable carbonate species were avoided by protecting neutral and alkaline solutions from air. To avoid interference from vanadium(IV), all systems were checked for paramagnetism by ESR spectroscopy. For a relevant computational treatment of the experimental data, the total concentration of each component has to be exactly known. Moreover, when quantitative multimethod data are collected, access to a computer program capable to treat such data is of vital importance. In the present work, the L A K E program has been used.
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5 1
Potentiometric Measurements. Potentiometric data were obtained from titrations and from separate pH measurements on individual samples. A l l titrations were carried out with an automated, computer controlled potentiometric titrator. The free hydrogen ion concentration was determined by measuring the emf of the cell: - A g , A g C l / 0.600 M/0.150MNa(Cl)//equilibrium solution /glass electrode + The measured emf (in mV) is expressed according to: £ = £ + 59.157-log [ H ] + £ j Ej is the liquid junction potential at the 0.600 M/0.150 M Na(Cl) // equilibrium solution interface and is, for the experimental setup used, given by EJmS? = -76/-331 [H ] + 42.5/143- K - [H ] for 0.600 M/0.150 M Na(Cl). K = 1.875 -10 /1.746 -10" is the ionic product of water in 0.600 M/0.150 M Na(Cl) at 25 °C (7,5). E is a constant determined separately in a solution with known [H ] immediately before and after each titration. When titrations were unsuitable (slow equilibria, spontaneous reduction) and for V N M R solutions, individual samples were prepared and pH measured with a combination electrode, which had been calibrated against buffer solutions of known [H ] in 0.600 M or 0.150 M Na(Cl). +
0
+
+
_1
w
14
14
w
+
Q
5 1
+
51
V NMR Measurements. Spectra were recorded at 131.5 MHz (11.7 T) and 25 ± 1 °C, using a Bruker AM-500 spectrometer. The chemical shifts are reported relative to the external reference V O C l (0 ppm). The field frequency stabilisation was locked to deuterium by placing the 8 mm sample tubes into 10 mm tubes containing D 0 . A 90° pulse angle was employed. Due to short relaxation times a relaxation delay of 10 ms 3
2
In Vanadium Compounds; Tracey, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
33 was sufficient. By careful phasing and baseline correction of the spectra, resonances could generally be accurately evaluated. Equilibria and Mass Balances. The equilibria studied are all written with the components H , H V 0 \ LH(1) and LH(2). LH(1) refers to the zero charged ligand of interest: alanylhistidine (AH), histidine (HIS), prolylalanine (PAH), alanylglycine (AGH), maltol (MaH), adenosine (AdH) or uridine (UrH2). LH(2) refers to zero charged imidazole (ImH). Thus, the complexes are formed according to: +
2
4
+
+
pU + ? H V 0 - + rLH(l) + sLH(2) ^ Downloaded by UCSF LIB CKM RSCS MGMT on August 28, 2014 | http://pubs.acs.org Publication Date: December 10, 1998 | doi: 10.1021/bk-1998-0711.ch002
2
(U ) (H V0 )^
4
p
2
(LH(l)) (LH(2)) M
4
r
(1)
S
m
Overall formation constants are denoted P q r,s & complexes are often given the notation (p,q,r,s) or V L(l)yL(2) ~. The total concentrations of vanadium, LH(1) and LH(2) are denoted [V] , [L(l)] and [L(2)] , respectively. By combining the law of mass action with the conservation equation, the total concentrations of each component are given by equations (2) to (5): Pf
t
n
x
z
tot
1
tot
tot
r
H= h - K h'
+ TSpfaqhPM + lXpP hPc
w
Ptr
a
s
r
YnLpP q hPb d P)
+ IZLpp hPc ds
)S
p>rfS
a
a
B = b + YLqp hPb
r
+ YSSppp^rhPlAcT +
PjS
a
r
+ YLY^pP Wb c d^
+ YZLqp hPbqd
P)q)r
(2)
pqj}S
s
+ YZLqp hPb c
p>q
s
+ YZpP hPd
+
p>q)S
r
YLYLqP
m°c ds
(3)
P)q>r>s
r
a
C = c + YZrp hVc
r
r
+ TLl>p hPb c
Pir
a
s
+ YZLrP hVc d
P)qir
+
pjfS
r
TLY^rP Wb c d^
(4)
PtqjiS
s
s
D = d+ ?ZsP hPd
+ ^YLsp q hPbQd
PiS
Pt
fS
r
s
+ ^YLsP r hPc d Pf
}S
+
JZZSspp^sWMfd?
(5)
+
H is the total concentration of H over the zero level of H 0 and chosen components. B, C and D are the total concentrations of vanadium and ligands, while h, b, c and d are the corresponding free concentrations. 2
Computer Programs. To obtain reliable deconvoluted integral data U X N M R / P version 1.1 and WIN-NMR version 950901.0 were utilized. To determine the stoichiometry (p,q,r,s) and corresponding formation constants (Pp,q,r,s) °f e complexes formed the least squares program L A K E was used (8). L A K E is able to calculate formation constants with standard deviations (3d) from, for instance, potentiometric data obtained in titrations or from separate pH measurements, quantitative integral N M R data, N M R shift data or combined potentiometric-NMR data. Formation constants for systematically chosen complexes (H )p(H V0 ~)
